Exotoxins produced by a wide range of bacterial pathogens are known to play a key role in host-pathogen interactions, establishment of the infection, and subversion of the immune responses. Treatment and prophylactic strategies targeting bacterial toxins are validated by decades-long history of multiple vaccines including tetanus, diphtheria, and pertussis. Monoclonal antitoxin antibodies for treatment of bacterial infections represent a promising strategy and this premise is strengthened by the recent FDA approval of Raxibacumab (anti-PA; GSK) for treatment of inhalation anthrax. In this Phase I SBIR project, we intend to devise a novel technology that allows specific targeting of neutralizing antitoxin antibodies to the site of infection where the antibody is needed. In this strategy, we exploit cell wall binding domains (CBD) of various phage endolysins, which bind with species-specificity and high affinity to bacterial cell wall components. Neutralizing antitoxi monoclonal antibodies will be fused to specific CBDs to generate Infection Site Targeted antitoxin Antibodies (ISTAbs). ISTAbs are expected to accumulate at the site of infection, where they capture and sequester toxins, thus immediately neutralizing pathogenic or immune suppressive effects of the toxins and prevent their release into circulation. The antibody-bacterium-toxin complex is then expected to be cleared by phagocytes. For creation of prototype ISTAbs, we target C. difficile, a major public health threat in light of increased antibiotic usage and the emergence of resistant strains, and a pathogen which relies heavily on toxin release to establish the infection. Glucosyltransferases toxin A (TcdA) and B (TcdB) are produced by virulent strains of this organism and play a key role in pathogenesis of C. difficile associated disease by targeting the Rho- GTPase family of cellular proteins. These toxins are validated as therapeutic targets in animal and human trials. We have generated a set of highly neutralizing antibodies to both toxins and produced them in a plant expression system. Thus, this pathogen and the two toxins provide an excellent model for proof of concept studies for ISTAb technology. The technology, if successful, will have far reaching applications to a wide range of bacterial pathogens where toxins play a key role in pathogenesis. In Aim 1 we will select the best CBD based on binding efficiency from a pool of different phage endolysins. In Aim 2 we will construct and characterize prototype ISTAbs using the best CBDs identified in Aim 1 and our neutralizing anti-TcdA and TcdB monoclonal antibodies. Phase II projection: Upon demonstration of proof of concept, we envision a Phase II SBIR focused on detailed efficacy studies that include combination with antibiotics, followed by process development and preclinical safety and pharmacokinetics.